The invention relates to a capacitive sensor, an associated evaluation circuit and an actuator for a motor vehicle according to the preambles of claims 3, 1 and 4.
Capacitive sensors are widespread not only in automation technology, but more recently also in the automotive industry, where they find application, inter alia, as a trunk or door opening or seat occupancy recognition.
DE 10 2012 106 526 A1 discloses a capacitive door handle sensor for a motor vehicle comprising at least two electrodes with different monitoring areas, wherein an electrode acts as a reference electrode (reference capacitance).
DE 10 2012 224 007 A1 discloses an arrangement and a method for converting the capacity of a measuring capacitor into a voltage which is detectable by an analog-digital converter comprising a charge transfer device for transferring the charge of an unknown capacitance Cx to a measuring capacitor CL, wherein, however, only the comparison of a single unknown capacitance Cx with a reference capacitance Cref is provided.
WO 2007 025 785 A1 discloses an evaluation circuit for capacitive sensors, wherein two capacitors are compared with each other. For this purpose two RC elements, each comprising one capacitor are supplied in parallel with the same square wave signal wherein their integration times, which due to their capacities are different, are discriminated by use of a XOR phase detector. The resultant square wave signal has a pulse-to-pause ratio which is dependent on the measuring value and is integrated by means of a further RC element so that an analog output signal is obtained. A disadvantage is that the circuit is not adapted for evaluating individual pulses but is only suitable for continuous operation.
AT 403 213 B discloses a similar working capacitive soil moisture sensor in which a time constant which depends on the capacitance to be measured results in a phase distortion of a square wave signal. Here, as a phase detector an AND gate is used, whose output signal is supplied to an RC filter element. The RC filter element converts the square wave signal into an analog signal which is dependent on its pulse-to-pause ratio. A disadvantage is that only one unknown capacitance (sensor electrode) can be measured at a time.